To fully appreciate the nature and breadth of the present invention, it is necessary to review efforts to develop precursors to silicon carbide, SiC. Two reviews, one directed to precursors to silicon containing glasses and ceramics, and the other to precursors to silicon carbide are: R. M. Laine and A. Sellinger, "Si Containing Ceramic Precursors", THE CHEMISTRY OF ORGANIC SILICON COMPOUNDS; Z. Rappoport and Y. Apeloig, eds., J. Wiley and Sons, London, in press 1997, pp. unknown; R. M. Laine and F. Babonneau, "Preceramic Polymer Routes to SiC," CHEM. MAT. 5, 260-279 (1993). These reviews are hereby incorporated as background information.
Ceramic precursor development requires attention to three interrelated criteria: synthesis, control of the polymer-to-ceramic process, and post-pyrolysis treatments. Thus, precursor development involves not only creating a simple and commercially viable synthetic approach; but also involves incorporating molecular architectures or elemental components that provide precursors with acceptable processability, high ceramic yield and environmental stability; generation of desired phase and chemical purity in the final ceramic product; and control of the average grain size and density of the final microstructure so the properties (physical, chemical and/or mechanical) of the resulting material can be tailored to specific applications.
For example, it is generally critical to incorporate controlled quantities of a sintering aid (e.g. 0.1-0.5 wt. % boron) in an SiC precursor, to provide for effective densification on sintering, without excessive grain growth, and without coincidental generation of extraneous phases. For example, at higher than desirable boron concentrations, formation of boron carbide changes the SiC stoichiometric ratio; changes the ceramic microstructure; and importantly, greatly increases the susceptibility of the ceramic product to oxidative degradation. All of these changes lead to loss of chemical, mechanical and electronic properties.
It is insufficient to perform well in only one of the three criteria previously discussed. To be "useful" for fiber forming, one of the most demanding processes in terms of precursor requirements, a precursor must offer: controllable rheology; latent reactivity;.sup.1 controllable pyrolytic degradation; high ceramic yield; high selectivity to desired ceramic product; and controllable densification and microstructural development. The following paragraphs briefly define the requirements of each category. The emphasis is on fiber processing, as fiber processing and products are the most demanding of precursor requirements.
Rheology. The type of fiber forming procedure used (extrusion from solution or melt, with or without drawing) places some constraints on what is considered useful polymer rheology. In general, the precursor should exhibit thixotropic or non-newtonian viscoelastic behavior such that during extrusion, the precursor may flow readily without necking. However, viscosity should be sufficiently high at zero shear so that the formed fiber retains its new shape and is self-supporting. Non-newtonian viscoelasticity normally arises in linear polymers with minimum molecular weights of 10-20 k Da as a result of chain entanglement. However, spinnable preceramics developed to date are low molecular weight, highly branched oligomers that exhibit non-newtonian behavior because the branches entangle. The correlation between rheology and "spinnability" has been discussed..sup.3,4
Latent Reactivity. Not only must precursor fibers be self-supporting as extruded, they must also remain intact (e.g. not melt or creep) during pyrolytic transformation to ceramic fibers. Thus, precursor fibers (especially melt spun fibers) must retain some chemical reactivity so that the fibers can be rendered infusible before or during pyrolysis. Infusibility is commonly obtained through reactions that provide extensive crosslinking. Such reactions include, but are not limited, to free radical, condensation, and oxidatively or thermally induced molecular rearrangements.
Pyrolytic Degradation. Most precursors contain extraneous organic ligands that are added to aid processability or provide latent reactivity. During pyrolysis, these extraneous ligands must be eliminated as gaseous products. The rates and mechanisms of decomposition to gases require close monitoring to ensure conversion to the correct ceramic material, to prevent retention of impurities or creation of gas generated flaws (e.g. pores). The processes involved can be likened to binder burnout in ceramic powder compacts..sup.5,6 In principle, this criterion is best satisfied if hydrogen is the only extraneous ligand required for stability and/or processability. Indeed, there is often a trade-off between precursor stability or processability and ceramic product purity that mandates processing with a less stable precursor to obtain higher quality ceramic products. In this instance, quality is equated with purity as detailed below. For example, Nicalon fibers derived from polycarbosilane, --[MeSiCH.sub.2 ].sub.n --, are not stoichiometric SiC because the original precursor is only processable with a 2:1 C: Si ratio (see below). More recently, polymethylsilane (PMS), --[MeSiH].sub.n --, was found to provide access to phase and chemically pure SiC fibers.
High Ceramic Yield. This criterion, which is product, rather than precursor-property driven, is critical to the design and synthesis of new precursors. The need for high ceramic yields arises because of the excessive volume changes associated with pyrolytic conversion to ceramic materials. Most precursor densities are close to 1 g/cm.sup.3, whereas most Si ceramic densities range from 2.5 to 3.5 g/cm.sup.3. The density of phase pure SiC is 3.2 g/cm.sup.3, for example.
For a ceramic yield of 100% (nothing is volatilized) and where complete densification occurs, the total volume change will still be .apprxeq.70%. However, a 100% ceramic yield is unrealistic. Even PMS, which in principle only needs to lose 2H.sub.2 molecules/monomer unit to form SiC, has a theoretical ceramic yield of 91 wt. %. A precursor with a ceramic yield of .apprxeq.50 wt. % (e.g. polycarbosilane).sup.1 will undergo volume changes of 85%. As a result, achieving near net shape in a final ceramic product becomes very difficult. Furthermore, the 50 wt. % of the precursor that leaves as gases can cause pores, uneven densification and leave behind entrapped impurities. Only in processing thin films or fibers can a 50% ceramic yield still be viable. In such products, mass transport and shrinkage are minimal in at least one dimension (in fibers, the diametrical dimension) and shape integrity can be retained..sup.1,2 Because diffusion distances for mass transport are very short (in fibers, the diametrical direction), gaseous byproducts can leave easily, permitting ready densification at higher temperatures. Finally, gaseous byproducts represent potential pollution problems that must be dealt with in commercial processes.
Hence, for most applications, high ceramic yield precursors are essential. Consequently, it is important to formulate a preceramic polymer that contains minimal amounts of extraneous ligands that allow it to meet the processability criterion and yet provide high weight percent conversions to ceramic product. Thus, hydrogen and methyl are the precursor ligands of choice, and reasonable ceramic yields typically range between 80-85 wt. %, because most precursor syntheses produce some quantity of low molecular weight species that evaporate rather than decompose during pyrolysis.
It is common to indicate ceramic yields at 1000.degree. C. For oxide ceramics, this temperature is usually acceptable. However, nonoxide ceramic precursors, especially SiC and Si.sub.3 N.sub.4 systems, often retain 1-2 wt. % hydrogen at temperatures up to 1400.degree. C. In 100 g of material, this corresponds to one mole of H.sub.2 per 2.5 moles of SiC if the desired end product is phase pure SiC. A 1000.degree. C. product with 2 wt. % hydrogen can be thought of as a solid solution of SiC and hydrogen. These materials will not exhibit the properties of phase pure SiC; because the hydrogen, which is most likely concentrated along the grain boundaries, does not permit normal microstructural development to occur. Furthermore, outgassing at higher temperatures can cause cracking, compositional changes or pores in ceramic shapes. Thus, care must be taken in reading and reporting ceramic yields.
Selectivity to Phase and Chemically Pure Glasses or Ceramics. Chemical and phase purity are critical issues that drive precursor design because optimal mechanical properties are achieved only with high purity. For example, ceramics grade Nicalon fibers, with a chemical composition of .apprxeq.SiC.sub.1.45 O.sub.0,36 and densities of 2.3-2.5 g/cm.sup.3,1,2 offer tensile strengths of 2.0-2.5 GPa and elastic moduli of .apprxeq.200 GPa. However, stoichiometric SiC fibers have densities &gt;3.1 g/cm.sup.3, tensile strengths of 3.0-3.5 GPa and elastic moduli of 400-470 GPa..sup.1,2 These values are equivalent to literature values for dense, pure SiC produced via standard ceramic processing methods. Despite the higher mechanical properties obtainable, however, chemical and phase purity are not always desirable. For example, H- and N-doped silicon carbide films behave as high temperature semiconductors, while silicon carbonitride glasses offer properties akin to glassy carbon with room temperature conductivities of 10.sup.3 (.OMEGA.-cm.sup.1).sup.7. Additional reasons for targeting materials that are not chemically or phase pure stem from the desire to control microstructural properties.
Control of Microstructure and Densification. As noted above, densified products provide optimal mechanical properties. Unfortunately, heating a precursor to high temperatures to convert it to phase pure material frequently does not lead to dense material. For example, precursor-derived phase pure SiC will crystallize and undergo grain growth on heating to 1800.degree. C. However, grain growth occurs without coincidental sintering (densification), leading to porous materials..sup.8 This problem can be solved by adding small amounts of boron (0.1-0.5 wt. %) which promotes densification without much grain growth. Thus, boron must be incorporated either during precursor synthesis or during processing, to achieve the desired microstructure. In this instance, microstructure drives precursor design.
In some instances, subtle changes in the precursor architecture can change the composition and microstructure of the final pyrolysis product. For example, pyrolysis of --[MeHSiNH].sub.x -- leads to amorphous, silicon carbide nitride (SiCN) solid solutions at &gt;1000.degree. C..sup.2 In contrast, isostructural --[H.sub.2 SiNMe].sub.x -- pyrolyzes to Si.sub.3 N.sub.4 /carbon nanocomposites on heating..sup.9 The properties of these two materials are quite different..sup.2
Essentially all non-oxide and many oxide ceramic fibers currently produced commercially are amorphous, or consist of nanocrystallites in an amorphous phase. Indeed, until recently the general consensus in the ceramics art was that crystalline fibers would fail more readily that glassy fibers because of flaws at grain boundaries..sup.10 Thus, much research has been directed toward the development of high temperature, glassy fibers with good mechanical and thermal properties. However, phase pure, microcrystalline SiC fibers provide properties and high temperature stability superior to current commercial fibers,.sup.1,2,11 and thus, nano- or microcrystalline fibers may be better in some or many applications, especially where creep is a problem.
In precursors targeted for coatings applications only, where the substrate provides most of the mechanical properties, additional criteria must be considered. First, the precursor must wet the substrate effectively to form uniform, adherent coatings. Some reaction with the substrate may or may not be desirable as a means of achieving either chemical and/or mechanical adhesion. Additionally, to process flaw (pore and crack) free ceramic coatings using dip, spin on, or spray coating processes; it is generally necessary to limit coating thicknesses to &lt;2 .mu.m and more commonly to &lt;1 .mu.m. This is because mismatches in coefficients of thermal expansion and the overall densification process lead to compressive stresses in the films. These stresses can provide improved coating adhesion and abrasion resistance; however, at higher thicknesses the compressive stresses cause coatings to crack, unless a ceramic powder is used as a filler to offset dimensional changes.
The same requirements for coating precursors applies to precursors used to process particulate and fiber reinforced composites by polymer infiltration followed by pyrolysis (PIP). Thus, wetting the surfaces of porous compacts or woven fiber preforms is of utmost importance to obtaining good matrix/reinforcement interfaces which are critical to achieving good mechanical properties. Because the surfaces of the particulates or fibers are often coated with unwanted materials, e.g. oxide layers, it is important to be able to adjust the stoichiometry to be a few percent rich in carbon or silicon to either cause reduction and elimination of the oxide layer or formation of a stable, unreactive oxide interface layer.
These general criteria serve as a basis for the selection of candidate precursors potentially of use for processing both oxide and nonoxide ceramics. For specific materials, additional criteria also play a role, including ease of synthesis and purification, and stability toward air and moisture. One final and critical criterion is cost. Costly syntheses can reduce the general utility of a given precursor. However, in ceramic fiber processing, the pyrolytic conversion and post-processing heat treatments designed to provide optimal fiber mechanical properties often contribute more to product cost than the chemistry of the precursors.
Historically, one of the first routes to a processable SiC precursor was that reported by Yajima et al..sup.12-14 in 1975 wherein polydimethylsilane was processed to produce SiC containing ceramic fibers. In the Yajima process, polydimethylsilane, obtained from dichloro-dimethylsilane monomer by condensation (dehalocoupling) with sodium metal, is rearranged at 470.degree. C. in Argon to yield Mark I PCS precursor, generally considered to have the formula: ##STR1## The precursor is melt-spun into fibers and oxidized in oxygen at 200.degree. C. to form cross-linked fibers which may be pyrolyzed in Argon at 1300.degree. C. to form SiC fibers having SiC.sub.1.45 O.sub.0.36 H.sub.0.003 stoichiometry in 60% ceramic yield.
This approach is still used to produce the only commercially available SiC precursor and SiC containing fibers: Nicalon.TM. fibers (Nippon Carbon, Sic.sub.1.45 O.sub.0.36 H.sub.0.03);.sup.1,12-14 Tyranno.TM. fibers (Ube Industries, SiC.sub.1.43 O.sub.0.46 T.sub.0.13);.sup.15-17 and Mark I PCS (Shin-Etsu Co.) precursor polymer..sup.10a As indicated by their respective compositional formulas, both Nicalon.TM. and Tyranno.TM. fibers are not phase-pure SiC. Thus, their properties are inferior to those of phase pure SiC:
TABLE 1 ______________________________________ Type Tensile Strength (GPa) Elastic Moduli (GPa) ______________________________________ Nicalon .TM. 2.0-2.5 &lt;300 Tyranno .TM. 3.0 &gt;170 SiC Whisker (single 8.0 580 crystal) Bulk SiC (not pressed) N/A 450 ______________________________________
In addition to not offering properties expected for phase pure SiC, the original Yajima process suffers from other drawbacks that include a multistep precursor synthesis and the inability to self-cure into fiber precursors of sufficient structural integrity such that they may subsequently be pyrolyzed. Finally, the presence of oxygen limits the upper use temperature for both Nicalon.TM. and Tyranno.TM. fibers to .apprxeq.1200.degree. C. because above this temperature CO and SiO gases evolve, generating defects (large crystallites, pores and voids) that contribute to substantial decreases in mechanical properties.
Because of these disadvantages, tremendous efforts over the past 20 years have focused on developing chemistries/processes to improve or replace the Yajima process. To date, two precursor types have been identified that transform to nearly phase pure SiC. These are polymethylsilane (PMS, (--CH.sub.3 HSi--).sub.n) and polysilaethylene, ##STR2## which are related in that on heating to &gt;300.degree. C., PMS transform via the Kumada rearrangement to polysilaethylene.
Both PMS and polysilaethylene or "polyperhydridocarbosilane" have a 1:1 Si:C ratio and in principle are designed to generate phase pure SiC. In practice, this is frequently not the case for a variety of reasons. Several other related precursor systems and processing methods that provide essentially phase pure SiC are also discussed below to provide perspective.
Two general routes are used to synthesize PMS: Wurtz dehalocoupling of CH.sub.3 HSiCl.sub.2 and transition metal catalyzed dehydrocoupling of methylsilane.
PMS via Dehalocoupling..sup.18-22 Seyferth et al. and Browning et al. described dehalocoupling of CH.sub.3 HSiCl.sub.2 with Na (Wurtz coupling) to synthesize PMS. CH.sub.3 HSiCl.sub.2 is added slowly to a mixture of Na sand in 7:1 hexane: THF with refluxing under Ar for 20 h. The polymer product can be isolated in 60%-70% yield as a viscous, hydrocarbon soluble liquid that gives a negative Beilstein test for Cl. .sup.1 H NMR characterization indicates Si-H ratios of 3.20:1 to 3.74:1, suggesting a co-oligomer composition consistent with --[MeHSi].sub.0.80 [MeSi].sub.0.20. Thus, 20 mol % of the original Si-H bonds are consumed by Na and/or reactive silyl intermediates (silyl radicals) resulting in Si atoms bonded to three other Si atoms. Thus, this type of oligomer contains few branches or cyclics and is substantially linear.
Molecular weights (M.sub.n) of the PMS oligomers range from 500 to 740 Da as determined cryoscopically (benzene), however, polydispersities were not reported. These oligomers are liquid and are spontaneously flammable on contact with air. The low molecular weights lead to low, 10-30%, ceramic yields (10 C/min./950.degree. C.). A typical ceramic composition on pyrolysis is Si.sub.1.42 C.sub.1.00 H.sub.0.14 l and thus not only contains a large excess of silicon, but contains considerable hydrogen as well. Other examples contained yet more silicon.
To increase ceramic yields, a reaction was run in straight THF, yielding a polymer suggested to be a co-oligomer with --[MeHSi].sub.-0.60, in 48% yield. However, no molecular weights could be obtained as this material was insufficiently soluble in benzene to permit cryoscopic measurements. It was suggested that this oligomer consisted of rings and chains with branching sites, where MeHSi groups were ring and chain members and MeSi groups were branch sites. A representative, discrete molecule had a methyl hydrogen to silicon hydrogen ratio of 3.6:1, and gave a 60 wt. % ceramic yield of a material with a stoichiometry SiC.sub.1.0 Si.sub.0.49, again far from stoichiometric. Slow evaporation of a toluene solution of this material gave a viscous residue from which fibers could be pulled. These precursor fibers, after photolysis in air for 1 h, could then be heated to 1000.degree. C. in N.sub.2 to give black ceramic fibers of unknown composition, the integrity of which was ascribed to formation of a thin SiO.sub.2 coating on the polymer fiber surface. The same fibers, following photolyzing in nitrogen, yielded only black powder. The researchers noted that results were "not especially promising."
To increase ceramic yields and carbon content, Seyferth et al. explored hydrosilylative crosslinking of PMS with cyclo- [CH.sub.3 (CH.sub.2 .dbd.CH) SiNH] 3 using catalytic amounts of AIBN. Reactions using SiH:SiCH.dbd.CH.sub.2 ratios .gtoreq.6 in refluxing benzene provided quantitative yields of soluble precursors with a 68-77 wt. % ceramic yield (1000.degree. C.). The pyrolyzed material exhibited a (SiC).sub.1.00 -(Si.sub.3 N.sub.4).sub.0.033 CO.sub.0.040 composition at 1000.degree. C. but at 1500.degree. C., the ceramic product was "at least partly crystalline." Only SiC was observed (XRD) to crystallize at this temperature. The 1000.degree. C. material is likely to be an SiCN composite. Related attempts to produce processable materials using trivinylsiloxane and trivinylsilathiane gave mixed results. Chain extension with the siloxane gave a polymer with M.sub.n .apprxeq.3k Da and 70.sup.+ wt. % ceramic yields but with considerable SiO.sub.2 and excess carbon in the product. The vinylsilathiane derivative gave low ceramic yields that were described as "less satisfactory".
Transition metal promoted dehydrocrosslinking of PMS has also been explored as an approach to increase ceramic yields and SiC purity. Because transition metals, e.g. Ru.sub.3 (CO).sub.12, are known to catalyze redistribution reactions between Si-H and Si-Si bonds,.sup.23 efforts were made to modify PMS via a chain-extension process to generate higher M.sub.n 's. For example, 1-2 wt. % Ru.sub.3 (CO).sub.12 added to PMS followed by irradiation for 4 h (140 watts at .apprxeq.300 nm) provided a polymer with a 55% ceramic yield of a Si rich material. To increase carbon content, Mark I PCS, [(--MeSiHCH.sub.2 --).sub.n ], was combined with PMS in a 1:2 wt. ratio and subjected to the Ru.sub.3 (CO).sub.12 catalyzed crosslinking. Pyrolysis of this polymer gave a 68% ceramic yield of high purity SiC with Si.sub.0.99 C.sub.1.00 stoichiometry. This approach suffers from being a multistep process and requiring the relatively rare metal ruthenium.
Metallocene (i.e. Cp.sub.2 ZrH.sub.2) catalyzed dehydrogenative crosslinking of PMS has also been considered as another method of increasing ceramic yields and carbon content during transformation of PMS to SiC:.sup.19-22. The addition of 0.6 mol % Cp.sub.2 ZrH.sub.2 to a hexane solution of PMS followed by refluxing for 2 h, resulted in loss of Si-H bonds, as determined by NMR, especially the SiH.sub.2 groups..sup.24 Pyrolysis to 1500.degree. C. gave ceramic yields of 70-80% with SiC purity as high as 98%, with ZrC and Si the primary contaminants in 1.6 and 0.4 wt. % respectively, SiC.sub.0.99 C.sub.100 Zr.sub.0.02. Unfortunately, fibers drawn from this PMS required UV curing for 1.5 h prior to pyrolysis to retain the fiber shape. Additionally, the metallocene catalysts renders PMS highly oxygen-sensitive and pyrophoric.
While Seyferths' work on metal catalyzed modification of PMS increased ceramic yields and improved product stoichiometry, the processes developed possessed numerous drawbacks as well, including starting oligomers that are not self-curing, and which in addition are pyrophoric; catalysts (Cp.sub.2 ZrH.sub.2) that are both expensive and pyrophoric; and the use of Ru catalysts, which are both expensive as well as being good oxidation catalysts, thus increasing the products' susceptibility to oxidation.
PMS By Catalytic Dehydrocoupling of MeSiH.sub.3. PMS can also be synthesized by catalytic dehydrocoupling of methylsilane, (MeSiH.sub.3) using Cp.sub.2 MMe.sub.2, (M=Ti,Zr) catalysts, as described by Harrod et al. and Laine et al..sup.24,26,27
The reaction is run in a toluene/cyclohexene solvent system at 25.degree.-60.degree. C. under .apprxeq.10 atm. MeSiH.sub.3 for 1-9 days using 0.2 mol % catalyst. The H.sub.2 byproduct is consumed simultaneously by metal-catalyzed hydrogenation of cyclohexene to cyclohexane, to minimize pressure build-up and depolymerization. PMS, for which .sup.1 H NMR suggests a partially branched structure, can be obtained in &gt;90% yield. SEC indicates M.sub.n .apprxeq.1200-1300 Da (DP.apprxeq.30) with a PDI.apprxeq.5-10. Pyrolysis (1000.degree. C./1 h/10.degree. C./min/Ar) provides a .apprxeq.77 wt. % ceramic yield of a material with a composition of Si.sub.1.0 C.sub.0.9 H.sub.&gt;0.2 O.sub.0.1 (6.0, 0.5 and 4.0 wt. % excess Si, H and O respectively)..sup.26,27 The oxygen appears to arise from handling. Note that percent excess silicon is considerably lower than that found on pyrolysis of the dehalocoupling reaction derived PMS materials which are, on average, half the molecular weight. Thus, nominal increases in molecular weight appear to be quite important in giving high ceramic yields and Si:C stoichiometries.
The PMS produced by the above process could be hand drawn or extruded to provide precursor fibers with diameters of 10-60 .mu.m. These fibers melt on heating to 90-100.degree. C. Fibers exposed to a nitrogen plasma, NH.sub.3, .gamma.- irradiation or UV for several hours survived heating to 1000.degree. C., but distorted during the pyrolysis process and were not deemed useful.
Further efforts to improve the infusibility of the fibers led to the direct introduction of vinylsilanes and silazanes as chain extenders designed to increase molecular weights such that the resulting PMS would decompose before it melted. Titanocene and zirconocene catalysts are known to promote hydrosilylation of terminal alkenes but not internal alkenes,.sup.24 thus efforts were made to directly introduce polyfunctional vinylsilanes to increase the molecular weight of the chains. However, introduction of .apprxeq.10 mol. % (20 wt. %) dimethyldivinylsilane to a typical PMS solution produced fibers which melted. Adding 10 wt. %. tetravinylsilane (TVS) provides infusible fibers that convert to phase pure stoichiometric SiC fibers.
Zhang et al. suggest that a lightly branched polymer structure results from addition of 10 wt. % tetravinylsilane (TVS) to PMS. However, the modified PMS still has insufficient molecular weight to produce spinnable materials. To obtain spinnable precursors, the modified polymer must be further heated to 80-90% of its gel time prior to reaction with TVS to get a spinnable fiber. The gel time for each batch of PMS must be determined empirically as the active catalyst concentration is very sensitive to any reaction impurities, e.g. chlorine or oxygen-containing materials. Thus, this step is long, arduous and dangerous because the precursor continues to be spontaneously flammable even after reaction with TVS.
Once the TVS has been added, the polymer can be spun directly from solution. Green precursor fibers can be spun and heated at rates up to 20.degree. C./min to 1000.degree. C. to form phase pure, SiC. However, the SiC crystallite sizes at this temperature are 2-4 nm (Debye-Scherrer), and the fibers retain excess hydrogen that can only be removed on heating to &gt;1400.degree. C. The fiber densities can be increased to nearly theoretical by heating to temperatures of &gt;1600.degree. C. in argon. Despite the ability to obtain phase pure material and full density, the dense fibers are actually quite porous, exhibiting very large crystallites (grain sizes of 1-3 .mu.m) with many large pores, resulting from sintering without densification. Given that boron, in very small quantities (typically 0.1-0.5 wt. %), is known to aid in sintering SiC,.sup.30-34 efforts were made to incorporate boron into TVS-PMS by hydroborating residual vinyl groups with a variety of boranes, including borane-methyl sulfide (Me.sub.2 S.multidot.BH.sub.3), borane-tetrahydrofuran (THF.multidot.BH.sub.3), borane-ammonia (NH.sub.3 .multidot.BH.sub.3), borane-trimethylamine (Me.sub.3 N.multidot.BH.sub.3), and borane-piperazine (C.sub.4 H.sub.9 N.sub.2 .multidot.BH.sub.3) complexes. Only the Me.sub.2 S.multidot.BH.sub.3 complex reacted with TVS-PMS to provide clear solution. All the other B-containing compounds precipitated out from the TVS PMS solution before or after volume reduction (under vacuum). Indeed, it appears that the catalyst is necessary to promote boron incorporation. The amount of boron incorporated was not known because it is difficult to control the reaction, but it was assumed to be on the order of 0.2 wt. %.
Although dense (3.1 g/cm.sub.3), phase pure fibers could be prepared from boron-containing TVS-PMS or TVSB-PMS, the process of preparing the precursors has numerous disadvantages: MeSiH.sub.3 is costly and pyrophoric, forming potentially explosive mixtures with ambient air; the reaction must be conducted under pressure to provide PMS; (3) the Ti/Zr dehydropolymerization catalysts used are pyrophoric; and (4) the metallocene derived TVSB-PMS is also highly pyrophoric. For example, exposure of TVSB-PMS samples to dry air results in rapid oxidation at room temperature as shown by TGA. Indeed, polymer samples ignite spontaneously in air. Further, disadvantages are that the synthesis is multistep and inconsistently reproducible because the catalysts used are very sensitive to impurities. Fiber spinning and all other types of precursor processing must be performed in to the complete absence of air until materials have been pyrolyzed to .apprxeq.1000.degree. C.
Tanaka et al..sup.35 also describe MeSiH.sub.3 dehydropolymerization to PMS in 68% yield, using CP.sub.2 NdCH(SiMez).sub.2 as catalyst. .sup.1 H NMR analysis showed a SiCH.sub.3 :SiH, ratio of .apprxeq.3:1 suggesting a linear polymer with no branching: SEC analysis gave M.sub.n =1470 Da with a PDI=5.00, very similar to the values reported by Harrod. Pyrolysis to 900.degree. C. in Ar provided ceramic yields .apprxeq.74%. with excess Si metal (amount not reported). To balance the excess Si, polyphenylsilane (Mw=1600 Da, 22 wt. %) was blended with PMS to give phase pure .beta.-SiC in 58% ceramic yield. The disadvantages to this process, in addition to those described above for MeSiH.sub.3, include: the necessity for use of Nd catalysts for large scale reactions; and (2) the low (58%) ceramic yields.
PMS By Catalytic Dehydrocoupling of CH.sub.3 SiH.sub.2 SlH.sub.2 CH.sub.3. Hengge et al..sup.36 studied Cp.sub.2 MMe.sub.2 (M=Ti,Zr) catalyzed dehydrocoupling of CH.sub.3 SiH.sub.2 SiH.sub.2 CH.sub.3. They report that Cp.sub.2 ZrMe.sub.2 catalytically dehydropolymerizes neat, liquid CH.sub.3 SiH.sub.2 SiH.sub.2 CH.sub.3 at room temperature in minutes to give an insoluble material with a general composition of H--[(MeSiH).sub.0.58 (MeSi).sub.0.42 ].sub.n --H. During reaction, the liquid initially turns yellow, then orange as H.sub.2 evolves vigorously and then gels. The resultant crosslinked polymer is insoluble in common solvents, decomposes before melting and is pyrophoric. Pyrolysis provides .apprxeq.88% ceramic yields (1500.degree. C./Ar) of SiC.
The Hengge process appears attractive because the starting dimer offers a 1:1 Si:C ratio, is a liquid, rather than gaseous CH.sub.3 SiH, polymerizes rapidly, and produces a ceramic product which appears to be phase pure SiC. However, the dimer starting material requires an expensive LiAlH.sub.4 reduction of CH.sub.3 SiCl.sub.2 SiCl.sub.2 CH.sub.3 which may be obtained from byproduct distillation of alkylchlorosilane "direct process" residue; and the resulting polymers offer no rheological utility: i.e. cannot be drawn into fibers, and are pyrophoric.
CH.sub.3 SiH.sub.2 SiH.sub.2 CH.sub.3 /(CH.sub.3).sub.2 SiHSiH.sub.2 CH.sub.3 mixtures can be copolymerized using Cp.sub.2 ZrMe.sub.2 as shown above for MeSiH.sub.3..sup.29 However, the resultant polymer gives low ceramic yields (50-60 wt. % at 1000.degree. C.) and is difficult to process. If the reaction is heated slowly to 250.degree. C., a pyrophoric yellow glassy solid forms in 15-20% yields, that is toluene soluble. LiAlH.sub.4 reduction of the redistribution product gives a polymer with a --(MeSi).sub.0.91 (Me.sub.2 Si).sub.0.09 (H).sub.0.4 -composition. Pyrolysis to 1560.degree. C. gave 90% ceramic yields of "essentially pure SiC", although no quantitative analyses were given. Given the above polymer composition, the theoretical yield to SiC would be 88.4%, suggesting the presence of excess carbon.
The advantages offered by this process are a one-step synthesis to poly(methylchlorosilanes) employing low cost dimers as starting materials; and fiber processing that leads to phase pure SiC fibers. The primary drawbacks to the process are the low precursor yield (15-20%); a multi-step process; a pyrophoric precursor; and the high cost of LiAlH.sub.4 reduction of the chlorinated polymer.
Polycarbosilanes as Precursors of SiC. Pillot et al..sup.41 reported the Wurtz polymerization of 2,4-dichloro-2,4-disilapentane (DCDP) to give a precursor to SiC. DCDP was synthesized via Mg/Zn coupling of dichloromethane with excess methyldichlorosilane in 35% yield. DCDP was polymerized via Wurtz coupling, followed by LiAlH.sub.4 reduction of residual Si-Cl, to poly(disilapentane) in 61% yield with Mn=1400 Da and PDI=3.1. On heating to 300-350.degree. C., poly(disilapentane) converts to the corresponding polycarbosilane (M.sub.n =7650 Da, PDI=3.0, softening point .apprxeq.245.degree. C.). Pyrolysis of this polycarbosilane gave ceramic yields of 79% (vs. 78% O. theoretical) with a 1.08 C:Si ratio and 1.1 mol %. The source of oxygen was not discussed, but is assumed to occur during handling. Direct pyrolysis of the polymer without prior heat treatments gave &lt;10 wt. % ceramic yields.
A related poly(disilapentane) precursor was developed by Corriu et al..sup.42 via metallocene catalyzed dehydropolymerization of 1,4-disilapentane (DSP). DSP was synthesized in 85% yield in two steps from the Pt-catalyzed hydrosilylation of vinylSiCl.sub.3 with MeSiHCl.sub.2 followed by LiAlH.sub.4 reduction. Reacting DSP with 0.5 mol % Cp.sub.2 TiMe.sub.2 at room temperature for 48 h gave a polymer with M.sub.n =900 Da and PDI=1.05. .sup.1 H, .sup.13 C and .sup.29 Si NMR analysis indicate formation of a linear polymer with no reaction at the resulting SiH.sub.2 sites. Reactions run for 72 h showed partial depletion of the SiH.sub.2 groups giving branched, highly viscous liquids with Mn=3260 Da and PDI=10.1. At 50.degree. C., crosslinking occurred rapidly to give an insoluble material within 30 min. Both low (Mn=990 Da) and high (Mn=3260 Da) molecular weight products gave high ceramic yields (&gt;73 wt. % yield vs. 78 wt. % theoretical) of nearly phase-pure SiC (Si.sub.1.01 C.sub.1.00) after pyrolysis. Conversion to crystalline .beta.-SiC commenced at 1100.degree. C. and was complete by 1400.degree. C., as determined by XRD. Ti was necessary during the pyrolytic transformation to SiC, as samples of poly(2,4-disilapentane) pyrolyzed without Ti gave 30% ceramic yields.
Phase Pure SiC via Processing. Many modifications to the existing Yajima polycarbosilanes (PCS) precursors have been reported in efforts to improve ceramic yields, phase purity and the mechanical properties..sup.1,2 Only a few of these actually provide phase pure SiC through processing efforts alone. For example, thermally stable, substantially dense polycrystalline SiC fibers (&gt;2.9 g/cm.sup.3 vs. 3.2 g/cm.sup.3 theory) can be processed from polydimethylsilane derived PCS by adding boron during the processing step..sup.45 PCS is first melt spun into fibers at .apprxeq.300.degree. C. and sequentially exposed to NO/diborane, or ammonia/BCl.sub.3 or NO.sub.2 /BCl.sub.3 gases (rather than O.sub.2) at temperatures between 25.degree.-200.degree. C. for periods of 4-24 h to render the fibers infusible. The residual N,O and excess carbon are eliminated during sintering, as gaseous byproducts (e.g. SiO and CO) at 1400.degree. C. resulting in pores and voids, that weakened the fiber. However, continued heating to &gt;1600.degree. C. results in smooth densification (decreasing porosity) and overall strengthening of the fiber.
The final, stoichiometric .beta.-SiC fibers have oxygen contents of &lt;0.1 wt. %, when heated &gt;1600.degree. C. The amounts of residual B are not discussed; although they could be as high as 5 wt..sup.46 The fibers exhibit average tensile strengths of 2.6 GPa and elastic moduli &gt;420 GPa, but lose 50% of their tensile strength on exposure to air at 1200.degree. C. for 40 h. Nicalon fibers also fail to survive similar heat treatments. The advantages to these fibers are that they rely on an existing process and provide properties expected of fully dense, phase pure SiC.
DeJonghe et al. have reported that simply heating Nicalon or Tyranno fibers at temperatures of .apprxeq.1600.degree. C. with sources of boron (e.g. boron metal, TiB.sub.2, etc.) leads to sufficient incorporation of boron into the fibers such that they densify and stable nearly phase pure SiC fibers are obtained, although no data is provided relative to fiber composition..sup.47 Toreki et al. have described processing SiC fibers (UF fibers) from a novel PCS with low oxygen content and better high temperature stability than Nicalon fibers..sup.48 The PCS was synthesized by pressure pyrolysis of polydimethylsilane in an autoclave. Fibers derived from PCS with Mn &lt;5000 Da melted before curing, while those derived from PCS&gt;10000 Da were not soluble, thus not suitable for spinning fibers.
In the presence of spinning aids (polysilazane and polyisobutylene), PCS of Mn.apprxeq.5000-10000 Da can be dry spun and the pyrolyzed in N.sub.2 to form SiC containing fibers. The polymer is self-curing, and provides fibers in 80% ceramic yields (950.degree. C., 20.degree. C./min.) . The fibers have low oxygen content (1.1-2.6 wt. %) and mechanical properties similar to Nicalon (tensile strength .apprxeq.3.0 GPa, ambient), but contain excess carbon, and thus perform below expectations for phase pure SiC. Sacks et al. have recently.sup.49 described near stoichiometric SiC fibers (.ltoreq.0.1 wt. % O) with high tensile strength (.apprxeq.2.8 GPa), fine grain sizes (.apprxeq.0.05-0.2 .mu.m), high densities (.apprxeq.3.1-3.2 g/cm.sup.3) with small residual pore sizes (.ltoreq.0.1 .mu.m). The synthesis of the polymer precursor for this process was not reported. However, it was stated that dopant additions were made The fibers (designated UF-HM fibers) retained .apprxeq.92% of their initial strength (2.70 GPa) after heat treatments to 1800.degree. C., suggesting the dopants contained B or other sintering agents to prevent grain growth. Electron microprobe analysis (EMA) showed an average fiber stoichiometry of Si.sub.0.93 C.sub.1.00.
As can be seen from the foregoing, both SiC precursors as well as precursor processing into SiC fibers, coatings, and other forms still require considerable improvements. It would be desirable to provide precursors which may be derived from relatively inexpensive starting materials without resort to use of expensive LiAlH.sub.4 reduction, or to expensive and pyrophoric organometallic catalysts. It would be especially desirable to provide precursors which are not pyrophoric, and which can be handled safely in normal ambient atmospheres. It would be yet further desirable to provide precursors which have rheologies suitable for spinning into fibers, for forming coatings, and for use as binders and infiltrants. It would be still further desirable to provide precursors whose chemistry may be altered to provide products having a targeted SiC ratio, optionally containing targeted amounts of additional elements and/or compounds.